iBooks update

I have the first iPad 5.1.1 my iBooks is now constantly installing. I understand this update is not compatible with my version. How do I get back to all the books I had before before this update occurred? They are backed up on my desktop but I cannot install the current version to restore them to it.

Install the iBooks version compatible to your device. Once you do that even if you have the books backed up on your computer you should also have them under the purchased section in your iTunes area or apps under purchased.